The purpose of this research proposal is to analyze the molecular mechanisms of somatic DNA recombination in lymphocytes. The recombination occurs in the gene system for the antigen-recognizing molecules of differentiating lymphocytes. Specific aims of this proposed project includes: 1) To further purify an endonuclease which specifically cleaves Ig DNA segments around the recombination site. 2) To isolate and characterize DNA binding proteins which recognize two consesus sequences at the recombination site. 3) To investigate a hypothetical RNA molecule which might hold two Ig gene segments together during recombination. 4) To establish an in vitro recombination system for V-(D)-J joining. 5) To establish a retroviral vector system to analyze the Ig gene recombination. 6) To examine the Ig heavy-chain and T-cell receptor B-chain genes for the formation of non-germline elements, which occurs at the joining sites of V, D and J segments. In addition to standard molecular cloning and DNA sequencing techniques, we will make use of the recombinant retroviral vector system for introducing defined Ig and T-cell receptor gene segments to various cell types of the lymphocyte lineages. It is hoped that the proposed research will contribute to our understanding of lymphocyte differentiation in molecular terms, but also to the clerification of complex recombination processes required for the activation of antibody genes.